Ethylene-vinyl acetate (EVA) foam material or thermoplastic polyurethane (TPU) foam material are widely used in making insole or outsole of shoes because of their superior cushion, shock-absorbing, heat insulation, moistureproof, chemical resistant properties. EVA and TPU are also nontoxic and non-water absorbing, which is quite environment friendly.
Referring to FIG. 1, a molding device 1 disclosed by Taiwanese Invention Patent No. 576329 includes a heating mold assembly 11 and a cooling mold assembly 12. The heating mold assembly 11 includes a lower mold 111, and an upper mold 113 that is removably connected to the lower mold 111 to cooperate with the lower mold 111 to define a mold cavity 112 therebetween. The heating mold assembly 11 further includes a plurality of heating members 114, such as resistive heater, that are disposed in the lower and upper molds 111, 113. When the heating members 114 are heated up, the lower and upper molds 111, 113 will also be heated up via thermal conduction to heat up a molding material received in the mold cavity 112.
The heating and subsequent cooling efficiency of the lower and upper molds 111, 113 will affect heating uniformity and heating cycle time of the molding material, and therefore the quality of a final product. While the resistive heating members 114 have a rather quick heating capability, which may be around 1 to 3° C. per second, it is quite difficult to precisely control the resistive heating members 114 to a desirable range, thereby resulting in difficulties in the temperature control of the lower and upper molds 111, 113, which might reduce the service life of the lower and upper molds 111, 113.